The present invention relates generally to fastening systems. In particular, the present invention relates to fasteners having ribs for improved transmission of rotational forces from driving devices, and for improved penetration into work pieces.
In the industrial, remodeling and do-it-yourself (“DIY”) markets for fasteners, such as self-tapping screws, fasteners having hexagon heads are widely used for a variety of applications. Such fasteners having hexagon heads can be forged for economical production in large quantities. The hexagon heads have six side surfaces joined by corners. An undesirable result of manufacturing hexagon head screws is that a large radius area is formed at the corners. This is caused because, during manufacturing, the corners cannot be fully filled during the forging operation. Such corners are referred to herein as rounded or under-filled corners.
Driving devices such as sockets or wrenches for driving the fastener are made to specific clearances with respect to the hexagon head of the fastener. Typically, a larger than desired clearance exists between the driving surfaces of the driving devices and the under-filled corners of the hexagon heads of the fasteners where the torque and rotational forces are transmitted from the driving devices to the fasteners. The rounded corners allow the socket to rotate or slip past the rounded corners of the hexagon head of the fastener. This causes a bursting stress between the rounded corners and the driving surfaces of the driving device. Bursting stress is the concentration of stress between the rounded corners of the hexagon head and the corresponding flat surfaces of the driving device when the driving device is rotated to drive the fastener. This leads to two types of failures: the driving surfaces of the driving device are worn quickly and continue to slip past the rounded corners of the hexagon head of the fastener. Subsequently the driving device may crack or otherwise fail due to the radial bursting stress. Also, the assembly secured by the fasteners can fail due to loss of constant torque from the driving device, due to the bursting stresses, which absorb some of the output torque and rotational forces. As a consequence, the seating torque of the various fasteners that hold together the assembly has a wide scatter. Subsequent degradation of the driving device results in the fastener head acting as a reamer inside the driving device and ultimately rotating freely inside the driving device, and the assembly torque is reduced to zero.
Also, in the same industries, carriage bolts with under head square portions are used. The square portions are used to prevent rotation of the bolt when it is tightened by a nut from the opposite side of the assembly. Completion of seating the bolt is made by drawing or pulling the square portion into the correct seating position in the work piece. The final seating position is normally achieved by tightening a nut onto the bolt from the other side of the assembly. Carriage bolts are typically tightened by a nut because such bolts usually have domed heads that do not provide any surfaces to be engaged by a driving device. This is due, for example, to the desire to have a low profile head.
Problems have been encountered with such carriage bolts when the conventional length of the under head square portion is not long enough to resist rotational forces transmitted by tightening of the nut. To compensate, the length of the square portions has been increased to at least the same size as the diameter of the bolt. This in turn presents another problem—it prevents the square portion from being fully driven and seated into the work piece. Often, the square portion does not have enough initial penetration into the work piece to resist the torque or rotational forces applied by the nut, which is tightened to draw or pull the square portion into final position in the workpiece. As a result of the minimal or shallow initial penetration of the square portion into the workpiece, the square portion rotates and reams the initial shallow penetration into a circular opening and ultimate assembly of the bolt fails. This situation is exacerbated when harder and higher density lumber, timber or composite material lumber are used.
What is needed are fasteners that overcome the above-discussed problems.